Switch to branchless normalization and extend EXCHANGE

Author: ucwong

core/vm/instructions.go

@@ -1167,24 +1167,34 @@ func opSelfdestruct6780(pc *uint64, cvm *CVM, scope *ScopeContext) ([]byte, erro
 	return nil, errStopToken
 }
 
+// decodeSingle decodes the immediate operand of a backward-compatible DUPN or SWAPN instruction (EIP-8024)
+// https://eips.ethereum.org/EIPS/eip-8024
 func decodeSingle(x byte) int {
-	if x <= 90 {
-		return int(x) + 17
-	}
-	return int(x) - 20
-}
-
+	// Depths 1-16 are already covered by the legacy opcodes. The forbidden byte range [91, 127] removes
+	// 37 values from the 256 possible immediates, leaving 219 usable values, so this encoding covers depths
+	// 17 through 235. The immediate is encoded as (x + 111) % 256, where 111 is chosen so that these values
+	// avoid the forbidden range. Decoding is simply the modular inverse (i.e. 111+145=256).
+	return (int(x) + 145) % 256
+}
+
+// decodePair decodes the immediate operand of a backward-compatible EXCHANGE
+// instruction (EIP-8024) into stack indices (n, m) where 1 <= n < m
+// and n + m <= 30. The forbidden byte range [82, 127] removes 46 values from
+// the 256 possible immediates, leaving exactly 210 usable bytes.
+// https://eips.ethereum.org/EIPS/eip-8024
 func decodePair(x byte) (int, int) {
-	var k int
-	if x <= 79 {
-		k = int(x)
-	} else {
-		k = int(x) - 48
-	}
+	// XOR with 143 remaps the forbidden bytes [82, 127] to an unused corner
+	// of the 16x16 grid below.
+	k := int(x ^ 143)
+	// Split into row q and column r of a 16x16 grid. The 210 valid pairs
+	// occupy two triangles within this grid.
 	q, r := k/16, k%16
+	// Upper triangle (q < r): pairs where m <= 16, encoded directly as
+	// (q+1, r+1).
 	if q < r {
 		return q + 1, r + 1
 	}
+	// Lower triangle: pairs where m > 16, recovered as (r+1, 29-q).
 	return r + 1, 29 - q
 }
 
@@ -1255,8 +1265,8 @@ func opExchange(pc *uint64, evm *CVM, scope *ScopeContext) ([]byte, error) {
 	}
 
 	// This range is excluded both to preserve compatibility with existing opcodes
-	// and to keep decode_pair’s 16-aligned arithmetic mapping valid (0–79, 128–255).
-	if x > 79 && x < 128 {
+	// and to keep decode_pair’s 16-aligned arithmetic mapping valid (0–81, 128–255).
+	if x > 81 && x < 128 {
 		return nil, &ErrInvalidOpCode{opcode: OpCode(x)}
 	}
 	n, m := decodePair(x)

core/vm/instructions_test.go

@@ -772,16 +772,7 @@ func TestEIP8024_Execution(t *testing.T) {
 	}{
 		{
 			name:    "DUPN",
-			codeHex: "60016000808080808080808080808080808080e600",
-			wantVals: []uint64{
-				1,
-				0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
-				1,
-			},
-		},
-		{
-			name:    "DUPN_MISSING_IMMEDIATE",
-			codeHex: "60016000808080808080808080808080808080e6",
+			codeHex: "60016000808080808080808080808080808080e680",
 			wantVals: []uint64{
 				1,
 				0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
@@ -790,30 +781,31 @@ func TestEIP8024_Execution(t *testing.T) {
 		},
 		{
 			name:    "SWAPN",
-			codeHex: "600160008080808080808080808080808080806002e700",
+			codeHex: "600160008080808080808080808080808080806002e780",
 			wantVals: []uint64{
 				1,
 				0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
 				2,
 			},
 		},
 		{
-			name:    "SWAPN_MISSING_IMMEDIATE",
-			codeHex: "600160008080808080808080808080808080806002e7",
+			name:    "EXCHANGE_MISSING_IMMEDIATE",
+			codeHex: "600260008080808080600160008080808080808080e8",
 			wantVals: []uint64{
-				1,
-				0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
-				2,
+				0, 0, 0, 0, 0, 0, 0, 0, 0,
+				2, // 10th from top
+				0, 0, 0, 0, 0, 0,
+				1, // bottom
 			},
 		},
 		{
 			name:     "EXCHANGE",
-			codeHex:  "600060016002e801",
+			codeHex:  "600060016002e88e",
 			wantVals: []uint64{2, 0, 1},
 		},
 		{
-			name:    "EXCHANGE_MISSING_IMMEDIATE",
-			codeHex: "600060006000600060006000600060006000600060006000600060006000600060006000600060006000600060006000600060006000600060016002e8",
+			name:    "EXCHANGE",
+			codeHex: "600080808080808080808080808080808080808080808080808080808060016002e88f",
 			wantVals: []uint64{
 				2,
 				0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
@@ -827,68 +819,31 @@ func TestEIP8024_Execution(t *testing.T) {
 			wantOpcode: SWAPN,
 		},
 		{
-			name:    "JUMP over INVALID_DUPN",
+			name:    "JUMP_OVER_INVALID_DUPN",
 			codeHex: "600456e65b",
 			wantErr: nil,
 		},
 		{
-			name:       "UNDERFLOW_DUPN_1",
-			codeHex:    "6000808080808080808080808080808080e600",
-			wantErr:    &ErrStackUnderflow{},
-			wantOpcode: DUPN,
-		},
-		// Additional test cases
-		{
-			name:       "INVALID_DUPN_LOW",
-			codeHex:    "e65b",
-			wantErr:    &ErrInvalidOpCode{},
-			wantOpcode: DUPN,
-		},
-		{
-			name:       "INVALID_EXCHANGE_LOW",
-			codeHex:    "e850",
-			wantErr:    &ErrInvalidOpCode{},
-			wantOpcode: EXCHANGE,
-		},
-		{
-			name:       "INVALID_DUPN_HIGH",
-			codeHex:    "e67f",
-			wantErr:    &ErrInvalidOpCode{},
-			wantOpcode: DUPN,
-		},
-		{
-			name:       "INVALID_SWAPN_HIGH",
-			codeHex:    "e77f",
-			wantErr:    &ErrInvalidOpCode{},
-			wantOpcode: SWAPN,
+			name:     "EXCHANGE",
+			codeHex:  "60008080e88e15",
+			wantVals: []uint64{1, 0, 0},
 		},
 		{
-			name:       "INVALID_EXCHANGE_HIGH",
-			codeHex:    "e87f",
+			name:       "INVALID_EXCHANGE",
+			codeHex:    "e852",
 			wantErr:    &ErrInvalidOpCode{},
 			wantOpcode: EXCHANGE,
 		},
 		{
-			name:       "UNDERFLOW_DUPN_2",
-			codeHex:    "5f5f5f5f5f5f5f5f5f5f5f5f5f5f5f5fe600", // (n=17, need 17 items, have 16)
+			name:       "UNDERFLOW_DUPN",
+			codeHex:    "6000808080808080808080808080808080e680",
 			wantErr:    &ErrStackUnderflow{},
 			wantOpcode: DUPN,
 		},
-		{
-			name:       "UNDERFLOW_SWAPN",
-			codeHex:    "5f5f5f5f5f5f5f5f5f5f5f5f5f5f5f5f5fe700", // (n=17, need 18 items, have 17)
-			wantErr:    &ErrStackUnderflow{},
-			wantOpcode: SWAPN,
-		},
-		{
-			name:       "UNDERFLOW_EXCHANGE",
-			codeHex:    "60016002e801", // (n,m)=(1,2), need 3 items, have 2
-			wantErr:    &ErrStackUnderflow{},
-			wantOpcode: EXCHANGE,
-		},
+		// Additional test cases
 		{
 			name:     "PC_INCREMENT",
-			codeHex:  "600060006000e80115",
+			codeHex:  "600060006000e88e15",
 			wantVals: []uint64{1, 0, 0},
 		},
 	}